A convex lens produces a double size real image when an object is placed at a distance of 18 cm from it. Where should the object be placed to produce a triple size real image ?

Answer

A, C

Explanation

Given `h_1=2h_0, h_1v/u`
`rarr v/u=2`
`rarr v=2u=36cm`
1/v-1/u=1/f`
1/36+1/18=1/f`
`rarr 1/f=3/36`
`rarr f=12 cm`
Now,` h_1=3h_0`
`h_i/h_0=v/u`
`rarrv/u=3 rarr v=3u`
`1/(3u)+1/4=1/12 rarr 3u=40`
`u=16cm`